High density bipolar electroplating electrolyte



Aug. 1, 1967 E. s. SNAVELY, JR, ETAL 3,33 ,031

HIGH DENSITY BIPOLAR ELECTHOPLATING ELECTROLYTE Filed May 18, 1964 EARLS. SNAVELY, JR.

WALLACE E. HARRELL,JR.

INVENTORS BY -QW United States Patent This invention relates to anelectrolyte; more specifically, to an electrolyte containing silver ionand having a high density.

Various silver-silver ion solution combinations have been used inelectroplating, primarily in conventional electroplating. Normally, suchsolutions contain cyanide ion. Such conventional electroplatingsolutions have a low density and are unsatisfactory, for one reason oranother, for bipolar electroplating.

Bipolar electroplating is often regarded as an undesirable side effectof conventional electroplating applications; however, for some purposes,it has been found desirable to utilize bipolar electroplating as theprincipal reaction in an electroplating system, minimizing as much aspossible any other efiects. In some applications it has been foundessential that the density of the electrolyte be quite highmuch higherthan is available in conventional electroplating solutions.

It is therefore a primary object of this invention to provide a highdensity silver plating solution having utility as an electrolyte forelectroplating applications wherein bipolar action is desired. It is afurther object of this invention to provide such an electrolyte that isstable under storage conditions and which is operative, for bipolarapplications, to yield a high quality deposit of silver on the bipolarelement interposed into the system.

More specifically, it is an object of this invention to provide anelectrolyte having almost 100% primary electrode efiiciencies and, atthe same time, having high bipolar etficiency.

This invention may have utility for various purposes wherever anelectrolyte is required; however, the primary application now known forthe invention is in the bipolar electroplating of certain elements. Aspecific currently known application is in the practice of a certainmicrobalancing method which is claimed in a copending application, Ser.No. 298,261, filed July 29, 1963, by Donald Luther Ensley, entitled,Ultrasonic Angular Displacement System, now Patent No. 3,293,919.

In the practice of such microbalancing method, an object to be bipolarplated in one case was a silver ring disposed about the periphery of aquartz inertial element in a flotation chamber. The inertial element wasto be balanced so that its center of buoyancy and center of mass wereessentially at the same location. To practically accomplish such aresult, the quartz inertial element assembly, including the silver ring,was placed within an electroplating cell with a plurality of silverelectrodes disposed concentrically about the outer periphery of thebalance ring. The electrolyte in the cell provided silver ion forplating action on the silver ring. By application of a voltage toselected pairs of electrodes, metallic silver was removed from oneportion of the balance ring adjacent cathode while silver was platedonto another portion adjacent an anode. The effective redistribution ofmaterial on the inertial ring, as the ring and the element rotate undergravity forces, balanced the inertial element by bringing the center ofmass and the center of buoyancy into coincidence.

In the conduct of such a microbalancing method it is important that theinertial element-ring assembly be approximately neutrally buoyant in theelectrolyte so that the assembly will not have a net unbalanced forceactice ing to cause it to sink in the electrolyte. Since the assemblyhas a specific gravity of about 2.6 (2.65 being a more precise figurefor the preferred assembly which is described in said copendingapplication), it follows that a suitable electrolyte for this specificapplication need have a specific gravity of at least about 2.6.

More generally, this invention has utility in any bipolar applicationwherein it is desired to transfer silver to an intermediate silvermember between two silver electrodes, and more specifically, wherein itis desired to effectively redistribute the mass of said intermediatesilver member by relatively increasing the silver mass of a part of saidintermediate member with respect to another part of said member.

The electrolyte of this invention may have utility for variousconventional (non-bipolar) applications and it is not intended herein todisclaim such other applications.

In its broadest aspect, the present invention provides a high densityelectrolyte comprising an acidic solution of silver iodide and a densecomplexing agent which is selected from the group consisting of bariumiodide and a mixture of indium iodide and lithium iodide. Such asolution has a specific gravity preferably in excess of about 2 atapproximately room temperature.

A quite specific preferred embodiment of the instant invention is a highdensity bipolar electroplating solution consisting essentially of anacidic solution of barium iodide and preferably of no less than about 1mole liter concentration of silver iodide, about 3 moles per liter beingparticularly desirable. The solution has a pH no higher than about 2 anda concentration of barium iodide which is complementary with thequantity of silver iodide selected to form a complex therewith and holdit in solution.'The specific gravity of this solution is at least ashigh as 2 and is preferably about 2.6 or above, not exceeding, ofcourse, the specific gravity possible under optimum conditions in such asilver iodide-barium iodide complex solution.

Another quite specific preferred embodiment of the instant invention isa high density bipolar electroplating solution consisting essentially ofan acidic solution of silver iodide containing lithium iodide and indiumodide, the latter reagent being added primarily to increase the specificgravity of the solution to a desired high value, and the former tocomplex the silver iodide. In this system, the pH is preferably nohigher than about 2 and the specific gravity is no less than about 2,the latter being desirably above about 2.6. In such system silver iodideis present in a concentration of at least about 0.1 mole per liter andpreferably at least as high as 0.25 mole per liter.

In the foregoing recited specific embodiments, lower alcohols,particularly ethanol and methanol, may be utilized as inert additives toincrease the stability of the solutions, to adjust specific gravity andto lower freezing point. Moreover, a minor amount, for example, 10 gramsper liter, of citric acid may be used in electroplating solutions ofthis invention to cause the silver deposited to have a smoother texture.

For a more complete understanding of the present invention, referencemay now be had to the following description taken in conjunction withthe accompanying drawings in which:

FIGURE 1 is a schematic side view of a bipolar cell, and

FIGURE 2 is an electrical diagram illustrating the electrical analog ofthe bipolar plating cell of FIGURE 1.

To more fully appreciate and understand the bipolar plating utility ofthe electrolyte of the instant invention,

a brief description of the general principles involved in bipolarelectroplating is offered.

If a metal electrode is placed in a solution containing Patented Aug. 1,1967.

ions of that metal, an electrical potential will exist between theelectrode and the solution. This potential is determined by severalfactors, two of which are the concentration of the metal ion surroundingthe electrode and the condition of the metal electrode surface. When asecond electrode is immersed in the solution and a voltage appliedbetween the two, electrolysis will occur. If the first electrode is madea cathode, a reduction process will occur at the metal-solutioninterphase in at least one of several ways. Either metal ion will bedeposited, the solvent will be reduced, or a combination of both willoccur, If more than one type of metal ion is present, one or more may bedeposited, depending on the voltage applied to the cell. Likewise, ifthe metal is made an anode, the metal may undergo dissolution, thesolvent may be oxidized, or again both may occur. In order to providebipolar plating that finds utility in the specific application referredto above, and more specifically described in the copending applicationmentioned, it is important that the solvent not be oxidized and thatmetal ions other than those of the electrode material not becodeposited. When a current is passed through a cell, compositionchanges occur in the solution near the electrodes and a potential changeresults. In addition, a resistive film may build up on the electrode andproduce an additional voltage drop. The total voltage drop across a cellat a given current is therefore made up of the voltage drop at the twoelectrode-solution interphases plus the LR. drop of the solution. Thedifference between the voltage across an electrode-electrolyteinterphase as a current passes and its equilibrium value is called theovervoltage. It is essential to keep the overvoltage as low as possiblein order to prevent side reactions, particularly for the specificapplication mentioned above. Overvoltage directly affects bipolarplating action.

Referring to FIGURE 1, 11 is a 150 milliliter glass beaker containing anelectrolyte 13. Non-conducting plastic cylindrical rod 15 of one inchdiameter is suspended concentrically downward from above the beaker withits lower end immersed in electrolyte 13. The lower immersed end of 15is recessed to receive and support identical semicircular bipolarelectrode segments B and B which abut to define a continuous ring.Segments B and B may be formed from 0.025 inch silver foil. Electrodes Aand C are adjacent, and identically spaced, for example A inch, from thecentral opposite portions of B and B, respectively. Electrodes A and Cmay be of various configurations. Those illustrated in FIGURE 1 areidentical curved electrodes, each representing about a 45 segment of athin annulus. They may be formed from 0.025 inch silver foil about onecentimeter square. Silver wires 17 and 19 are connected, as by spotwelding, to A and C, respectively. A suitable DC. voltage source (notshown) is provided for 17 and 19, with 17 connected to the anode and 19to the cathode. The preferred electrolyte for bipolar plating in thecell of FIG- URE 1 is the silver electroplating solution of thisinvention.

Bipolar plating is a result of a parallel low resistance path betweenthe two main electrodes in an electrolysis cell. For bipolar plating, ofhigh efficiency, it is desired to effectively allow a transfer of massfrom the area of B to B when a voltage is applied between A and C (FIG-URE 1), Normally, as in the silver ring for the inertial elementdescribed in the copending application and briefly referred to above,areas B and B are opposite areas on a continuous mass of metal, but inthe case illustrated the strip has been divided into the oppositeabutting bipolar electrodes for convenience of reference. An electricalanalog of the cell shown in FIGURE 1 is represented in FIGURE 2.Therein, E and E are the anode and the cathode overvoltages,respectively, and E and E are the overvoltages of the respectiveportions of the bipolar element. They vary logarithmically with current.In general, E and E are not equal to'E and E since a parallel path isavailable. E is the LR. drop across the electrolyte between the mainanode and cathode and is linear with current. The character F representsa solution resistance that is smaller than E since it includes only theelectrolyte between A and B and B and C. Current flow may proceed eitherdirectly from A to C via an ionic transfer through the electrolyte 13 orfrom A to B via ionic transfer, followed by electrochemical reaction atB to convert to electronic conduction, followed by an electronictransfer from B to B followed by another electrochemical reaction at Bfollowed by ionic transfer from B to C. The path chosen, or, moreaccurately, the ratio of current flowing through one path to the other,will depend on the ratio of the respective voltage drops. The degree ofbipolar plating action is enhanced by minimizing E and E by providing anelectrolyte of low conductivity to maintain a high E and by maintaininga close electrode spacing to make E substantially smaller than E Since Band E are always smaller than or equal to E and E the problem then is inproviding an electrode-electrolyte system of low anode and cathodeovervoltage, preferably one which will plate from a low conductivitysolution. For the specific application discussed previously herein andexplained in more detail in the above-mentioned copending application,it is additionally essential that the density of the solution be ofsufficiently high value to suspend a practical inertial element withinit, or alternatively, to be slightly heavier than said inertial element,on a per volume basis, in order that the inertial element will be buoyedup somewhat.

The problem of providing a high density bipolar electroplating solutionis a difficult one because of various side reactions, undesirable sideeffects, and low solubilities.

The present invention will now be described, first in relatively generalterms as applied to one specific embodiment, followed by three specificexamples relating thereto, and then by a general description relating toanother specific embodiment, followed by an example relating to thatembodiment.

A preferred embodiment of the instant invention is a silver iodidesolution, complexed by barium iodide, and adjusted to have the desiredspecific gravity. Such silver iodide solutions may be obtained, having ahigh specific gravity, one substantially in excess of 2.0.

Barium iodide is sufiiciently soluble in water to form a solution havinga specific gravity of 2.28 at room temperature. A saturated solution ofbarium iodide dissolves sufificient silver iodide to increase specificgravity to 2.75 at room temperature. Such a solution can, in turn, beutilized to dissolve additional barium iodide. This property of bariumiodide to complex silver iodide makes it possible to formulate anelectroplating solution having a high specific gravity, for example, oneof 2.65. This is most desirable for the specific utility of the instantinvention which was described in the preceding portions of thisapplication. In formulating dense solutions of silver iodide, complexedwith barium iodide, it is desirable to maintain sufficient acidity toprevent precipitation of the compound BaI -BaO-9H O; however, if thesolution is too acid, corrosion results on the electrode material.

The following examples illustrate satisfactory solutions of silveriodine, complex by barium iodine. While these examples are generally fora density of about 2.65, since since this was the area of intenseinterest for specific application of the instant invention, densesolutions having specific gravities in the range from about 2 to 2.6 maybe obtained by proper treatment, as is mentioned in the examples, andthese resulting solutions function satisfactorily for bipolar plating atsuch densities. It is preferred that the specific gravity of thesolution obtained be at least as high as 2. It is additionally preferredthat the concentration of silver iodine be at least as high as about 1mole per liter.

Example 1 A solution A is prepared as follows: 110 grams of reagentgrade Ba(OH) 8H O' are dissolved in 100 milliliters of purified 48%hydriodic acid. Because of the variations in concentration of commercial48% hydriodic acid, it may be necessary to add more acid drop-wise todissolve all the Ba(OH) '8H O or to add more of the latter if too muchacid is present. The resulting barium iodine solution is evaporateduntil a specific gravity of 2.13 is obtained, at which point thesolution has a boiling point of approximately 116" C. The pH is adjustedto about 2 by addition of Ba(OH) -8H O, -or hydriodic acid, as required.Small quantities of iodine formed are removed by refluxing the solutionwith silver metal. The resulting solution obtained is Water-white.

A solution B is prepared as follows: 24 grams of silver iodine aredissolved in 25 millimeters of Solution A to form a solution having aspecific gravity of 2.70.

A plating solution of desired specific gravity may be obtained bycombining appropriate quantities of Solution A and Solution B.

An electroplating solution obtained by such combination, found to havesuperior stability and to be excellent for bipolar plating applications,is in ratio of two parts of Solution A to eigth parts of Solution B. Itwill be observed that this is a silver iodide concentration of about 3.3moles per liter. The density of the resulting mixture is about 2.65. Ifa higher density is desired, it may be obtained by adding barium iodide.On the other hand, the density may be decreased by adding water. Theaddition of a small amount of water causes a yellow precipitate, whichmay be dissolved by stirring.

The plating solution described above, having a specific gravity of 2.65,was tested by passing current through a simple cell having silverelectrodes, the current density being about 2 milliamps per squarecentimeter. The anode of the cell was found to have lost weight by 15.2milligrams and the cathode tohave gained weight to the extent of 15.1milligrams. This is to be compared to a theoretical change of 14.8milligrams to be transferred from the anode to the cathode.

The texture of the silver deposited from the solution described may bemade smoother by utilizing approximately 0.1 gram of citric acid permilliliters of solution as an additive.

On concentrating the solution described aboveto about 2.75 by boiling,then diluting with ethanol to obtain a density of approximately 2.65, aplating solution was obtained, 25 milliliters of which are utilized fora bipolar plating test in the apparatus of FIGURE 1. The resistivity ofthis solution was on the order of 50 ohm-cm. About 80% bipolar platingefiiciency with 100% electrode efiiciency is obtained.

Current flow of from 1 microamp to 10 milliamps is preferred for bipolarplating with the above solution.

Example 2 Solutions A and B, prepared as described in Example 1 abovewere mixed in ratio of 1 milliliter of A to 9 milliliters of B. A simpletest conducted by passing current through a cell containing thissolution resulted in an anode loss of 15.1 milligrams and a cathode gainof this same amount. This is to be contrasted to the theoretical of 14.8milligrams transferred.

It is to be noted that the density of this solution was adjusted to2.65, as previously described.

Bipolar plating tests utilizing the mixture of this example givesubstantially the results of those obtained in Example 1.

Example 3 The various procedures of Example 1 were repeated, but thistime using a plating solution having 3 milliliters of Solution A mixedwith 7 mlliliters of Solution B. The concentration was adjusted to 2.65by the method previously described.

It is found that substantially the same results are obtained on a simpletest when current density of 2 milliamps flows through the solution fromanode to cathode. Bipolar plating tests yielded similar results to thoserecited in Example 1.

Once again, it is found that the addition of a small amount of citricacid, about 0.1 gram per 10 milliliters of solution, for example,promotes the formation of a smoother deposit of silver.

Silver iodide may likewise form a satisfactory electroplating solutionof high density when complexed with lithium iodide to which has beenadded a compatible agent to increase density, idium iodium beingpreferred for this purpose. The silver iodide concentration may be low,but it is preferred that it be as high as 0.1 mole per liter, with about0.25 mole per liter representing a desirable level.

As was the case with the silver iodide-barium iodide solutions, it isnecessary to keep the solution acid in order to preventprecipitationprobably of In(OH) (I) The following example describes aspecific silver iodidelithium iodide-indium iodide system.

Example 4 Seventy grams of indium metal are dissolved in 300 millimetersof 48% hydriodic acid. The reaction, which is slow at room temperature,is accelerated by refluxing the mixture. If desired, the reaction ratemay be increased by refluxing in the presence of spongy indium. Spongyindium may be formed by pouring the molten metal in cold water.

The resulting solution obtained from the dissolution of the indium metalin hydriodic acid has a specific gravity of 1.8. It is evaporated untila specific gravity of 2.7 is obtained, which occurs at a boiling pointof about 112 C. Thereafter, lithium iodide (LiI) is added to thesolution in concentration of about 2.5 moles per liter (335 grams perliter). Since commercial anhydrous lithium iodide contains up to about30% water, it is highly desirable to dry the lithium iodide by heatingit in a vacuum before the use described above. The dried lithium iodideso obtained is quite hygroscopic and must be protected from theatmosphere.

Finally, silver iodide is dissolved in the indium iodidelithiumiodide-hydriodic acid 'water system to a concentration of about 0.25mole (59 gm.) per liter. The resulting solution is a bright yellow andmay be adjusted to a specific gravity of 2.65 by diluting with water,acidified to a pH of about 2.5, or by adding crystalline indium iodide.

In practicing the foregoing procedure, it has been noted that thelithium hydroxide content of commercial lithium iodide varies widely andmay neutralize the hydriodic acid excessively. This condition isevidenced by a difliculty in dissolving lithium diodide atthat stepwhere it is added to the solution to a concentration of 2.5 moles perliter. If, for some reason, the ratio of lithium iodide to silver iodideis too low, a second liquid phase will separate from the indium iodidesolution. This condition may be remedied by adding more lithium iodideto the solution. It is preferable that the acidity of the solution bemaintained below a pH of 2 at all times to prevent precipitation,probably of In(OH) (I) If such precipitation does occur, it may beremedied by refluxing with excess hydriodic acid to redissolve theprecipitate. Then excess hydriodic acid must be removed, as by reactionwith metallic indium.

Plating experiments, as those described in the examples for the silveriodide-barium iodide complex system (Examples 1-3) were conducted on thesolution prepared in accordance with the foregoing procedure. It wasfound that plating efliciencies of substantially resulted. In contrastto silver iodide-barium iodide solutions, slight corrosion occurs whensilver metal is stored in a solution made in accordance with thisexample; however, the amount of such corrosion is not critical for mostpurposes. Weight loss through corrosion does not exceed about 1milligram from a 2 square centimeter area of silver in contact with thesolution for hours.

The bipolar plating elliciencies for the solution described above wereof the same order tobtained for the silver iodide-barium iodide system.

The specific gravity of the foregoing system may be adjusted upwardly ordownwardly by addition of indium iodide or water, respectively.

Silver iodide, complexed either with barium iodide or with a lithiumiodide (with indium iodide added to increase density) functionssatisfactorily as a plating electrolyte over wide concentration ranges(for example, as low as 0.01 molar AgBr). However, with the specifictype application that is now being made of the electrolyte, it isimportant that it be quite dense, with a specific gravity (roomtemperature) of at least about 2 and preferably of about 2.6.

Of the two systems described, the barium iodide complexing systemappears to be the more desirable. It is quite stable. For example, thesolution of Example 1 can be stored for two weeks without any change inits appearance. When chilled, the solution is in equilibrium withcrystalline material at 10 C. It is thus seen that wide ranges ofoperation and high stability are provided by this solution. In addition,the silver iodide-barium iodide system provides a substantiallycolorless solution, being essentially water-white, and is thus highlytransparent. Prolonged exposure to air causes the slightest amount ofturbidity, which can be removed quickly by filtration. The translucenceof an electrolyte prepared in accordance with silver iodide-bariumiodide is an additional important characteristic when the solution isutilized as a bipolar plating solution to practice the microbalancingmethod described in the above-mentioned copending application.

Having described the invention in connection with certain specificembodiments thereof, it is to be understood that further modificationsmay now suggest themselves to those skilled in the art and it isintended to cover such modifications as fall within the scope of theappended claims.

What is claimed is:

1. A high density aqueous electrolyte comprising an acidic solution ofsilver iodide and a dense complexing agent, said complexing agent beingselected from the group consisting of barium iodide and a mixture ofindium iodide and lithium iodide, said solution having a specificgravity in excess of 2.

2. The electrolyte of claim 1 wherein said dense complexing agent isbarium iodide.

3. The electrolyte of claim 1 wherein said dense complexing agent is amixture of indium iodide and lithium iodide.

4. The electrolyte of claim 1 wherein said solution has a pH of nogreater than about 2.

5. The electrolyte of claim 4 wherein said dense complexing agent isbarium iodide.

6. The electrolyte of claim 4 wherein said dense complexing agent is amixture of indium iodide and lithium iodide.

7. The electrolyte of claim 1 wherein said density is in excess of about2.6.

8. The electrolyte of claim 7 wherein said dense complexing agent isbarium iodide.

9. The electrolyte of claim 7 wherein said dense complexing agent is amixture of indium iodide and lithium iodide.

10. A high density aqueous bipolar electroplating solution consistingessentially of an acidic solution of silver iodide and a densecomplexing agent, said complexing agent being selected from the groupconsisting of barium iodide and a mixture of indium iodide and lithiumiodide, said solution having a specific gravity of about 2.6 and a pH ofabout 2.

11. A high density aqueous bipolar electroplating solution consistingessentially of an acidic solution of barium iodide and silver iodide,the latter in concentration of no less than about one mole per liter,said solution having a pH of no greater than about 2 and a barium iodideconcentration complementary to the silver iodide concentration to holdthe silver iodide in solution and provide an overall solution specificgravity of at least about 2.

12. The solution of claim 11 wherein a lower alcohol is in the solution.

13. The solution of claim 11 wherein a minor proportion of citric acidis present as a smoothing agent for silver deposit.

14. The solution of claim 11 wherein the silver iodide concentration isabout 3 moles per liter.

15. The solution of claim 14 wherein the specific gravity is about 2.6.

16. A high density aqueous bipolar electroplating solution consistingessentially of an acidic solution of silver iodide, indium iodide andlithium iodide, the concentration of silver iodide being at least 0.1mole per liter of solution and the concentration of indium iodide andlithium iodide being selected to hold the silver iodide in solutionwhile providing an overall solution specific gravity of at least about2, the pH of said solution being no higher than about 2.

17. The solution of claim 16 wherein a lower alcohol is in the solution.

18. The solution of claim 16 wherein a minor proportion of citric acidis present as a smoothing agent for silver deposit.

19. The solution of claim 16 wherein the silver iodide concentration isabout 0.25 mole per liter.

20. The solution of claim 19 wherein the specific gravity is about 2.6.

References Cited UNITED STATES PATENTS 5/1932 Schlotter 204-46 2/1957Nobel et al 204-46 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPatent No 3 ,334 ,031 August 1 1967 Earl S. Snavely, Jr., et al.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 2 line 28 after 'mole" insert per line 42 for "odide" read iodidecolumn 3 line 47 for "B" read B column 4 line 3 for "P read E samecolumn 4 lines 64 and 74 and column 5 lines 8 and 17 for "iodine", eachoccurrence, read iodide column 5, line 17, for "millimeters" readmilliliters line 25, for "eigth" read eight line 73, for "mlliliters"read milliliters column 6, line 13, for "idium iodium" read indiumiodide line 25 for "millimeters" read milliliters line 53, for "diodide"read iodide column 7, line 4, for "tobtained" read obtained Signed andsealed this 15th day of April 1969.

(SEAL) Attest:

EDWARD M.PLETCHER,JR. EDWARD J. BRENNER Attesting Officer Commissionerof Patents

1. A HIGH DENSITY AQUEOUS ELECTROLYTE COMPRISING AN ACIDIC SOLUTION OFSILVER IODIDE AND A DENSE COMPLEXING AGENT, SAID COMPLEXING AGENT BEINGSELECTED FROM THE GROUP CONSISTING OF BARIUM IODIDE AND A MIXTURE OFINDIUM IODIDE AND LITHIUM IODIDE, SAID SOLUTION HAVING A SPECIFICGRAVITY IN EXCESS OF 2.